The ISO 13406-2 method to measure the monitor’s response time as the total time necessary to change the state of a pixel from pure black to pure white and back again brings but very little information about the real performance of the monitor and easily misleads the user. Today we are going to reveal all secrets about this parameter and discuss the response time compensation that affects the quality of dynamic images on the screen of 7 monitors from ViewSonic and Samsung.

Although each monitor manufacturer promotes it under a proprietary trademark, the response time compensation concept has come to be known in the press and among the users as “overdrive”. This is a kind of a misnomer because the term “overdrive” refers only to the boost impulse of positive polarity as illustrated by the first figure. If the impulse is negative, as on the second figure, it is referred to as “under-drive”. So, the correct technical term is Response Time Compensation and it combines both under-drive and overdrive. Unfortunately, I have already witnessed how the term overdrive misleads people into thinking that LCD monitors only accelerate the transitions towards lighter tones, accomplished with a positive boost impulse, but this is not true. In all available monitors with response time compensation both types of compensation are implemented!

It should also be made clear that the RTC mechanism works on the lowest level possible. It processes the signal that then goes directly to the matrix. The fact is the value of the overdrive impulse depends only on the current position of the liquid crystals and the position they must be turned into. All the user-adjustable settings have to be made before the RTC block or else the RTC block would have to correct the compensation value depending on the user-defined parameters (contrast, color temperature, etc) which would be technically complex, but not really called for.

The RTC mechanism thus belongs entirely to the monitor and has no relation whatever to the graphics card, the driver, the OS or any other external object. If a monitor has RTC, RTC will work all the time, even if the monitor finds itself connected to a Tseng Labs ET-4000 graphics card on a computer running MS-DOS 5.0. If a monitor does not have RTC, it is next to impossible to emulate it with the graphics card since the emulation algorithm would have to be modified after any change in the user-adjustable settings of the monitor. Just take the 100 grades of contrast available on a typical modern monitor and the 100 grades of each of the basic colors (I mean the manual color temperature setup) and some 5 gamma compensation exponents, and you’ll get as many as 100*100*100*100*5 = half a billion variants! It would be virtually impossible to adjust the RTC emulation algorithm for each of the variants.

Software RTC emulation by the graphics card driver began to be discussed after such technology was mentioned in the descriptions of ATI’s and NVIDIA’s mobile graphics processors. The authors of the “sensation” just didn’t have a clear understanding of the RTC concept as well as of the difference between desktop and notebook monitors. The notebook’s own monitor makes up a single whole with the graphics subsystem and does not have its own settings. On a notebook, it is through the graphics chip that all the image setup operations are performed, so the requirements “the RTC block must be located after all the user-adjustable settings” and “the RTC block is in the graphics chip” are not contradicting. A desktop LCD monitor always has its own settings which can not be controlled by the graphics card – even the models that are controlled through software, like the Samsung SyncMaster 173P, have them.